RESPONSES OF RAT NUCLEUS SUBMEDIUS NEURONS TO ENKEPHALINS APPLIED WITH MICROPRESSURE

The purpose of this study was to determine what effects leucine-enkephalin and D-Ala2-D-Leu5-enkephalin have on both the background and naturally evoked activity of thalamic nucleus submedius neurons responsive to mechanical cutaneous stimulation. Thirty-five neurons in the nucleus submedius were fully characterized during single-unit extracellular recordings as nociceptive, low-threshold mechanoreceptive (LTM) or unresponsive. Micropressure was used to apply the opioids. Eighteen neurons were inhibited; 13 of these were nociceptive and one was LTM. Six units were activated; two of these were nociceptive and three were LTM. The remaining 11 units were unaffected. Opioid responses were tested for antagonism by naloxone in 12 neurons; eight of these responses were antagonized by naloxone. Statistical analyses indicated that the effects of enkephalins on nociceptive neurons were selective for neuronal modality. The opioids also altered the response of some nociceptive neurons to receptive field stimulation. The presence of nociceptive neurons in the nucleus submedius that are selectively inhibited by opioids provides additional support for the involvement of submedius neurons in nociception. The results of this study suggest that this involvement is more than merely transmission of nociceptive input, since the opioids may be selectively modulating the type of information that is transmitted to the cortex.